1. Field of the Invention
The present invention relates to a dental radiology apparatus and a method for using this apparatus.
2. Description of the Related Art
In the field of dental radiology, radiology apparatuses are known which comprise an X-ray generator and an X-ray sensor each mounted on an arm with an arch-shaped structure, for example within the framework of acquisitions of panoramic or cone beam tomographic images.
There are apparatuses which allow only panoramic photographs to be taken and others which are capable of producing both panoramic images and cone beam tomographic images.
In the first type of apparatus, the X-ray generator is provided with a vertical collimation slit and the sensor made in the form of an array of pixels is placed behind a vertical slit arranged opposite the slit of the generator.
The object to be radiographed is placed between the X-ray generator and the sensor. The X-rays are emitted by the generator in the shape of a cone collimated by the slit in the direction of the object. The sensor receives the rays that have illuminated the object, converts them into electrical signals and provides at the output an image signal of the illuminated object.
When it is desired to produce, with this type of apparatus, a radiograph of a patient's jaw and in particular to obtain a panoramic image of it, the patient is placed in a sitting or standing position under the arch, between the generator and the sensor.
The arch pivots about a vertical axis of rotation while the jaw is illuminated by the X-rays in order that the sensor can provide image signals of the patient's jaw.
Simultaneously with the rotation movement, the axis of rotation of the arch describes a trajectory in the shape of a horseshoe which follows the shape of the patient's dental arch. The zone of interest under investigation (dental arch) covers the jaw.
However, as the generator and the sensor are placed on either side of the patient, during the rotation bony structures will be superposed on the jaw in the resultant image captured by the sensor.
In order to limit the effects of this superposition, the pixels of the sensor are shifted at a rate which is governed by the movement and position of the arch. This gives rise to a kinetic blurring such that the undesirable bony structures mentioned above manifest themselves in horizontal bands (streaking) which limit the inconvenience when establishing a diagnosis.
Moreover, the procedure is such that the pixels of the sensor which are read shifted in the opposite direction to the movement of travel along the trajectory remain fixed in relation to the plane of the arch located in the clear zone.
This technique, which also contributes to a better dosimetric balance, is known under the name of TDI (“Time Delay Integration”).
The combination of the rotational speed of the arch, the trajectory of the center of rotation and the translation speed of the pixels on the sensor allows a focal trough to be obtained which has the shape of a band of greater or lesser width which follows the patient's dental arch. The objects located on either side of this focal trough manifest themselves in streaking.
In the resultant image which presents itself as a developed image of the patient's jaw, the objects located in the focal trough will stand out clearly from the streaking caused by the objects located on either side of this zone, thus facilitating diagnosis.
The displacement of the sensor and the generator along this trajectory is achieved using a servo-motor mechanism (for example an X,Y-controlled table) placed above the arch and controlled to steer the movement along the X- and Y-axes in the plane of rotation of the sensor and the generator.
The control of this mechanism requires knowledge of the trajectory matched to the shape of the patient's dental arch.
When producing panoramic pictures, the operator of the apparatus does not have available information allowing him to control the displacement of the sensor and the generator in a manner suitable for the patient's jaw.
Thus, the apparatus very often makes several standard forms of dental arch available to the operator from which he selects the one that seems to him to be the most suitable for the patient to be radiographed. These standard forms are based on statistical data of typical morphologies. The apparatus is then programmed in order that the assembly formed of the sensor and the generator travels the trajectory corresponding to the selected standard form of arch (the trajectory is defined as being the median line between the two opposite edges of the standard form of the dental arch).
However, this solution is not entirely satisfactory, since the sensor and the generator are not positioned in a manner matched to the morphology of the patient's jaw.
Problems of clarity may result for the zone of interest in the image obtained in this way. For example, the patient's teeth cannot be completely registered in the standard form of arch selected by the operator.
Moreover, the selection process which has just been described requires the operator to carry out several maneuvers in order to obtain a panoramic image which, moreover, is sometimes vague over certain zones of the dental arch (incisors, molars . . . ).